Preparation of photomasks

ABSTRACT

The present invention relates to the preparation of an x-ray photomask by exposing a free-standing film of a radiation sensitive metal/chalcogenide to an electron beam scanned in a defined pattern so as to generate areas in the film of reduced metal content in accordance with the defined pattern, as well as novel x-ray photomasks.

[0001] The present invention relates to the preparation of an x-rayphotomask for use in the manufacture of, for example, silicon chips. Theinvention also relates to novel x-ray photomasks.

[0002] To produce microchips of high density, smaller components andsmaller interconnects between the components are required. Integratedcircuits are currently etched onto silicon chips using aphotolithographic process. This process involves covering the microchipwith a suitable resist, exposing the resist through a patterned mask toUV light, and finally removing the unexposed areas by etching in asuitable solvent. The wavelength of UV sources has now become a limitingfactor for the dimensions of integrated circuits [i], as well as thedimensions of photomask architecture.

[0003] Sulphur, selenium and tellurium are elements of group VI of theperiodic table and are collectively known as the chalcogen elements. Thename chalcogen stems from the Greek word khalkos (χαλκóç) for copper andrefers to the occurrence of these elements in copper ores [ii].Compounds that contain one or more of the chalcogen elements are calledchalcogenides. In amorphous form the chalcogenides possess a variety ofdistinctive properties that lend these materials for manifoldapplications in the production of electronic devices. Currently, theyfind their main application as high-resolution resists in photo andelectron beam lithography. Amorphous chalcogenides exhibit a number ofinteresting photoinduced effects. These range from minor structuralrearrangements, which result in changes in the optical bandgap energy,to more complex atomic and molecular modifications, such asphotocrystallisation or photodissolution of metals [iii].

[0004] One of the most interesting properties of the amorphouschalcogenides is the tendency of certain metals to dissolve in thesematerials under illumination with light. This effect is known asphotodissolution or photodoping and can be observed inchalcogenide-metal layer structures. The result is a basis for manyapplications of the chalcogenides, for example as inorganic resists inphoto and electron beam lithography.

[0005] An interesting phenomenon has been observed when photodopedchalcogenides are illuminated through a mask with visible or UV light[iv]. The metal content increases in the illuminated region anddecreases in the covered region. This effect was first reported byTanaka et al. and has become known as photoinduced chemical modification(PCM). Tanaka et al. and Yoshida et al. [v,vi] studied the effect mainlyin Ag—As—S glasses.

[0006] When a thin film of amorphous Ag—As—S is partially covered andilluminated with light, silver migrates from the covered region towardsthe illuminated region. The process can be reversed by covering thepreviously illuminated area and illuminating the sample again. Thesilver migrates towards the illuminated area and a reversed Ag profileis obtained. When the cover is removed and the whole sample isilluminated, the silver redistributes homogeneously throughout thesample

[0007] Electron beam irradiation of Ag-photodoped chalcogenides has beenshown to cause a migration of silver towards the irradiated area,[vii-viii]. Although this electron induced chemical modification (CM)appears to be similar to the PCM, there are some significantdifferences. The metal, once accumulated in the irradiated region doesnot redistribute under repeated irradiation and the effect seems to bemuch more efficient when induced by an electron beam. An increase insilver content of up to 20 at. % in the irradiated region has beenobserved [vi].

[0008] Yoshida et al. [vi] reported a dependence of ECM on theaccelerating voltage. The greatest compositional changes are observed atan accelerating voltage of 10 kV. Further increasing of the acceleratingvoltage results in a decreased magnitude of ECM. At acceleratingvoltages higher than 50 kV no ECM generated increase of Ag or only avery small increase is observed in the irradiated region. The ECM isgenerally accompanied by a volume expansion of the irradiated region.Features of 5 μm height and diameter have been produced by Yoshida etal. [ix].

[0009] It should be noted that under certain experimental conditions afocused electron beam causes silver to diffuse away from the irradiatedregion. This effect has been observed by McHardy et al. [x] in freestanding chalcogenide-silver films. Silver free lines of 15 nm widthhave been produced using a high resolution scanning transmissionelectron microscope.

[0010] The present invention is based in part on the phenomenon thatmetal diffuses away from an irradiated region in free standing silverdoped chalcogenide films and as identified by the present inventors howthis may be employed in making x-ray photomasks.

[0011] In a first aspect the present invention provides a method ofpreparing an x-ray photomask for use in the production of semiconductordevices, the method comprising the steps of:

[0012] a) providing a free-standing film of a radiation sensitivemetal/chalcogenide; and

[0013] b) exposing the film to an electron beam scanned in a definedpattern in order that metal in the metal/chalcogenide diffuses away onexposure to the electron beam so as to generate areas in the film ofreduced metal content in accordance with the defined pattern.

[0014] The method may further comprise the step of

[0015] c) removing the chalcogenide in the electron-beam exposed area toleave a film comprising metal/chalogenide in the unexposed areas andmetal of reduced content or substantially metal free content in theexposed areas.

[0016] The metal/chalcogenide film is “free-standing” in the sense thatfilms are not supported on a bulk substrate. Rather, the films may besupported on for example grids or membranes, such as silicon nitridemembranes, of thicknesses between for example 50 nm-150 nm, typically100 nm. The metal/chalcogenide film is typically an amorphouschalcogenide film in contact with a metal such as silver, copper,platinum, palladium or the like. Any metal which is able to react withthe chalcogenide such as by illumination and diffuse away on exposure toa suitable radiation beam may be used. Suitable chalcogenides includeAs₂S₃, As₂Se₃, GeS, GeSe, and GeSe₂, and other non-stoichiometricchalcogenides of arsenic, germanium, bismuth and antimony.

[0017] It is possible to first provide a layer of chalcogenide and coatthis with a layer of metal. Subsequently reaction of the metal, forexample by illumination, results in the metal reacting with thechalcogenide and the two layers becoming less distinct. Alternativelythe metal may be coated as a layer first, followed by the chalcogenide.Typically the layers are prepared by vacuum evaporation. However, themetal can also be absorbed into the chalcogenide film as ions from asolution. The film then behaves in a similar way in an electron beam asmetal/chalcogenide bilayers formed by vacuum deposition. Generallyspeaking the metal concentration in the metal/chalcogenide film may begreater than 30 wt. %, preferably greater than 50 wt. % such as 70 wt.%.

[0018] The film is exposed to an electron beam scanned in a definedpattern. The electron beam of suitable energy (ie. 15 keV to 100 KeV),may be focused to produce a fine point of radiation capable of drawinglines for example using an e-beam writer on the metal/chalcogenide ofless than 10 μm, 5 μm, 1 μm, 100 nm and even down to less than 50 nm, 10nm or 5 nm thickness.

[0019] The metal in the metal/chalcogenide diffuses away on exposure tothe radiation beam to leave an exposed area which comprises metal ofreduced metal content or preferably substantially metal free.

[0020] By using for example an e-beam writer focussed to a fine beam itis possible to draw extremely complex patterns of reduced metal contentor substantially metal free (ie. by exposure to the beam) in themetal/chalcogenide film. Areas with significant amounts of metal content(eg. A bilayer containing a 500 nm thick film of silver) absorb x-rays,whereas areas that have reduced metal content or are substantially freeof metal transmit x-rays.

[0021] Depending on the thickness and/or x-ray transmissibility of thechalcogenide, the chalcogenide may be removed to leave a film comprisedof metal/chalcogenide in unexposed areas) and areas of reduced metalcontent or substantially metal free in exposed areas. The chalcogenidemay be removed for example using an alkaline solution such as a 0.3NNaOH solution. The film may thereafter be supported by an x-raytransmissive substrate, such as silicon nitride or carbon.

[0022] The present invention also relates to a photomask manufactured bya method as described herein.

[0023] In a further aspect the present invention provides a photomaskcomprising an x-ray transmissive substrate supporting a metal filmcomprising areas of metal which areas are x-ray non-transmissive andareas of reduced metal content, or substantially metal-free which areasare x-ray transmissive.

[0024] There is also provided use of a photomask as described herein inthe manufacture of an electronic device, especially a semiconductordevice.

[0025] According to a yet further aspect of the invention there isprovided use of a radiation sensitive metal doped chalcogenide in thepreparation of a photomask.

[0026] The present invention will now be further described by way ofexample and with reference to the figures which show:

[0027]FIG. 1 shows a TEM image from a pattern in an As₂S₃/Ag film of 33nm thickness and 51 wt. % Ag, produced by an electron beam of 100 kV ina JEM 200CX;

[0028]FIG. 2 shows an AFM image from a pattern in an As₂Se₃/Cu film with38 wt. % Cu, produced in an electron beam writer at 15 kV acceleratingvoltage and 1 nA beam current; and

[0029]FIG. 3 shows a section analysis of a pattern on As₂Se₃/Cu filmwith 38% Cu.

[0030] Sample Preparation

[0031] All materials were deposited by physical vapour deposition (PVD)to produce samples in thin film form that could be analysed bytransmission electron microscopy. Silicon nitride membranes in the formof TEM compatible frames, and frames for x-ray masks were supplied byFastec Ltd. These frames contained thin film windows with membranes of100 nm thickness.

[0032] Evaporation of Chalcogenides and Metals

[0033] Bi-layers of a chalcogenide and a metal were prepared by physicalvapour deposition (PVD). Two kinds of sample were produced, referred toas chalcogenide/metal layers and metal/chalcogenide layers in referenceto the order of evaporation.

[0034] It should be noted that silver and copper diffuse into and eventhrough the chalcogenide layer, as demonstrated by Fitzgerald et al. [x,xi]. These metal and chalcogenide layer therefore become less distinctimmediately after preparation.

[0035] The metals were evaporated from resistance heated tungsten boats.Tungsten pepperpot boats were required for the evaporation of thechalcogenides to prevent these materials from “jumping” out of the boatupon heating. All materials used are given in Table 1 with theirrespective purity, form and evaporation rate. The evaporation rate mustbe carefully controlled as it was found that at high evaporation rates,the films exhibited inaccurate stoichiometry and tended to peel off thesubstrates. TABLE 1 Properties of the evaporated materials Chemicalpurity Form rate Material (%) Evaporation (nm/min) As₂Se₃ 99.999 fusedpieces 26.8 As₂S₃ 99.999 down fused lump 37.1 Ag 99.99 wire, 0.25 mm Ø12 Cu 99.99 foil, 0.127 mm 14.3 Au 99.99 wire, 0.2 mm Ø 6.6 In 99.9shot, 4 mm Ø 17.4

[0036] The film thickness was monitored in situ using a quartz crystal.This crystal is excited into thickness shear mode vibrations of 6 MHz byan external oscillator. The actual frequency of the crystal'soscillation depends on the mass deposited on its surface and decreasesas the deposit builds up. This change in frequency is proportional tothe mass of the deposited material and can consequently be used tocalculate the film thickness [xii].

[0037] The majority of samples consisted of a metal film of 5 nm to 16nm thickness and a chalcogenide film of 23 nm to 100 nm thickness. Thiscorresponds to metal proportions ranging from 8 wt. % to 48 wt. %, whilethe overall mass of the films remained constant. However, films with ametal content of up to 70 wt % were studied.

[0038] Patterning of Films

[0039] The majority of the films were patterned using an electron beamwriter, which allowed setting and monitoring of all exposure parameters,such as acceleration voltage, beam current and exposure time. Theelectron beam writer consists of a commercial scanning electronmicroscope (JOEL JSM-T220) that can be computer controlled using thesoftware ELPHY Quantum, Universal SEM based Nano Lithography System,Version 1.232.

[0040] The microscope can be operated at accelerating voltages between 5and 30 kV in 5 kV increments. No line growth could be observed at anaccelerating voltage of 5 kV; and at 30 kV the patterns appeared blurryin the SEM image for most samples. To study the influence of theexposure conditions on the growth characteristics, lines of about 50 μmlength were drawn using the line scan mode. The accelerating voltage wasincreased from 10 to 25 kV in 5 kV increments, while the exposure timewas varied between 2 min and 15 min per line at each of the accelerationvoltage settings. The beam current for this experiment was set at 1 nA,which was the highest beam current attainable at the lowest acceleratingvoltage. The sets of lines were written onto thin films of As₂Se₃/Ag,Ag/As₂Se₃, As₂Se₃/Cu and Cu/As₂Se₃. At least four samples of each kindwere prepared and studied.

[0041] To establish whether As₂Se(S)₃/In and As₂Se(S)₃/Au films can bepatterned by an electron bear, a number of samples with varying metalcontent were exposed to an electron beam using the line scan mode at 15kV accelerating voltage and 1 nA beam current, which had produced goodresults in the preceding experiments.

[0042] A number of films supported on thin film membranes were patternedin a conventional transmission electron microscope, which providedhigher accelerating voltages. The accelerating voltage was set at 100kV. The electron beam was focused to a spot size of 1.4 μm and manuallyscanned across the sample by moving the specimen shifting knobs.

[0043] The size of the patterns was subsequently measured employing anatomic force microscope (AFM. It was shown that at low accelerationvoltages (10 kV to 20 kv), with films deposited on substrates, most ofthe electron-hole pairs are excited within the first few 100 nm, whichcorresponds to the thickness of the films.

[0044] Transmission Electron Microscopy

[0045] Transmission electron microscopy (TEM) is a powerful technique inthe study of the microstructure of thin films. In this project a JEOLJEM-100C and a JEOL JEM-200CX instrument have been employed to study themicrostructure of thin films of chalcogenides and metals. Thesemicroscopes provide accelerating voltages of up to 100 kV and 200 kV,respectively. The JEM-200CX is equipped with a Gresham energy-dispersivex-ray spectrometer (EDS), which allowed compositional analysis of thematerials.

[0046] Both instruments can be operated in imaging mode or indiffraction mode.

[0047] A Nanoscope, Dimension 3000 scanning probe microscope (SPM) fromDigital Instruments was used to obtain three-dimensional images ofpatterns on chalcogenide-metal films.

[0048] Atomic force microscopy (AFM is a very powerful tool to imagesurface topography on a nanometre scale. It facilitates the measurementof surface features in all three dimensions, with a z resolution ofbetter than 1 Å.

[0049] A very sharp tip (of the order of a few nanometres radius ofcurvature at the end) is scanned across the surface of the sample thusprobing and mapping the morphology of the surface [xiii]. The tip ismounted on the end of a long cantilever that has a spring constant lessthan the interatomic bond strength.

[0050] For investigations the instrument was operated in tapping mode toavoid damage to the surface. This was especially important when patternson thin film membranes were measured. In this mode of operation thecantilever is oscillated at a frequency close to its resonance frequency(about a few hundred kHz) while retracted from the surface. The tip isthen brought towards the sample until it touches the surface. The vander Waals forces between tip and surface atoms cause a decrease in theoscillation amplitude. The new amplitude is retained constant by meansof a feedback control loop while the tip is scanned across the surfaceby a piezoelectric scanner that can be moved in sub-angstrom increments.The scanner adjusts the tip height to maintain a constant height overthe surface. These height adjustments are monitored using a beamdeflection system. A laser beam is focused onto the back of thecantilever and is reflected from there onto a segmented photodiode.Minute deflections of the cantilever cause one segment of thephotodetector to collect more light than another. The amplified signalis proportional to the deflection of the cantilever and is used to imagethe forces across the sample.

[0051] Amorphous chalcogenide thin films in contact with silver, copper,gold or indium films were studied. The materials were prepared by vacuumevaporation and were subsequently investigated using electrondiffraction and x-ray microanalysis (EDS) in the transmission electronmicroscope (EM). The aim was to confirm the stoichiometric compositionof the films, to determine the level of reaction with the chalcogenidesand to ascertain which reaction products could be formed. The results ofthis study provide the basis for investigations of the patterns that canbe produced in these materials by electron beam irradiation.

EXAMPLE 1 Amorphous Chalcogenides and Silver

[0052] The contact reaction of As₂Se₃ and As₂S₃ with Ag was studied byelectron diffraction in the TEM.

[0053] Bi-layers of chalcogenide and silver with silver contents between8 wt. % and 70 wt. % were prepared and analysed. It was found thatsilver films dissolve spontaneously into chalcogenide films. Noadditional illumination with light was necessary to induce thisreaction. It was also not important whether the metal was evaporatedunderneath or on top of the chalcogenide.

[0054] Samples with high silver concentrations (>50%) were verysensitive to electron irradiation and it was found that the silverdiffused away from the irradiated areas. This could be observed in thefollowing way: the microscope was operated in the diffraction mode andthe sample was moved to an unexposed area. A diffraction pattern wasobserved (data not shown). Within a few seconds these patterns changedand the diffraction pattern from a pure chalcogenide was observed.

[0055] It should be noted here that to avoid silver diffusion whiletaking diffraction patterns, the intensity of the electron beam had tobe reduced to a very low level. The microscope was set up at normalintensity, then the intensity was reduced drastically and the samplemoved to a fresh area. From this new area the diffraction pattern wasrecorded.

EXAMPLE 2 Amorphous Chalcogenides and Copper

[0056] Bi-layers of amorphous chalcogenides and copper with copperconcentrations between 8 and 48 wt. % were prepared and analysed usingelectron diffraction in the TEM.

[0057] Similar to silver, copper was found to react readily withamorphous chalcogenides. Films containing copper in high concentrationsare sensitive to electron beam irradiation. Copper diffused away fromthe irradiated area under prolonged irradiation at normal intensity. Theeffect was not as strong as in chalcogenide-silver films, but the sameprocedure as described above for obtaining diffraction patterns wasemployed. In the case of chalcogenide-silver and chalcogenide-copperfilms that contained high proportions of silver or copper, a diffusionof these metals away from the irradiated area was observed uponirradiation with an electron beam of high intensity. This effect was notobserved in the investigated chalcogenide/indium films.

EXAMPLE 3 Formation of Lines by Electron Beam Irradiation

[0058] A focused electron beam was scanned across the surface of afree-standing chalcogenide-silver film in the TEM. The silver diffusedaway from the irradiated area. The effect occurred only in samples witha silver content of more than ≈40 wt. % for As₂Se₃/Ag films and morethan ≈45 wt. % for As₂S₃/Ag films. When operating the microscope in thediffraction mode this diffusion effect could be clearly observed: thesample was moved to a fresh area and initially the diffraction patternof the reaction product was visible on the screen. In a few seconds thisdiffraction pattern changed and was replaced by the diffraction patternof the pure chalcogenide. FIG. 1 shows a TEM image of a patterngenerated in the film by manually moving the sample using the specimenshifting knobs while irradiating the sample with a focused electron beamof 100 kV accelerating voltage.

[0059] A diffraction pattern, taken from the irradiated area, confirmedthat the silver had diffused away from the irradiated area. Only thediffraction pattern of amorphous As₂S₃, with a very sharp firstdiffraction ring, was detected.

[0060] Similar to silver, a copper diffusion in As₂Se(S)₃/Cu films wasinduced by a high intensity electron beam. The effect occurred inAs₂Se₃/Cu films containing more than ≈30 wt. % Cu, and in As₂S₃/Cu filmscontaining more than ≈35 wt. % Cu.

[0061] Chalcogenide-gold and chalcogenide(indium films containing highlevels of gold and indium did not exhibit the diffusion effect. Thediffraction patterns did not alter, even under prolonged exposure with ahigh-intensity electron beam.

EXAMPLE 4 Investigation of Line Patterns

[0062] Chalcogenide-silver and chalcogenide-copper films withsufficiently high metal concentrations were exposed to a focusedelectron beam in an SEM. The accelerating voltages applied were 10 to 30kV. Utilising the line scan mode, the electron beam was scanned acrossthe films and line patterns formed. These lines were analysed in theTEM, and the diffraction patterns indicated that the silver and copperhad diffused away from the irradiated areas. This signified that theeffect also occurs at much lower accelerating voltages than usuallyapplied in the TEM.

[0063] To determine whether the lines formed in the SEM producedprotruding patterns in the films, some of the samples were investigatedusing an atomic force microscope (AFM. To provide a better stability forAFM measurements, chalcogenide-silver and chalcogenide-copper films ofsufficiently high metal concentrations were deposited onto siliconnitride membranes of 100 nm thickness. They were then patterned using anelectron beam writer, operated at 15 kV accelerating voltage and 1 nAbeam current. An example of a pattern produced in this way is displayedin FIG. 2. The three-dimensional image shows that troughs had formed inthe film. From the section analysis of the pattern, presented in FIG. 3,the depth of the troughs was measured and found to be about 34 nm andthe width was found to be about 2.5 μm.

[0064] In films of very high silver or copper concentration ahigh-intensity electron beam causes the metal to diffuse away from theirradiated area. By scanning a focused electron beam across these films,troughs were formed, which did not contain silver or copper.

[0065] One of the important features of the diffusion effect is that itis limited to the irradiated area Hence, the size of the pattern isrelated to the spot size of the electron beam. Using a medium spot sizein the transmission electron microscope JEM 200CX silver-free troughs ofa width of about 200 nm were produced on an As₂S₃/Ag sample with 51 wt.% Ag. The spot sizes used in the electron beam writer were usually muchlarger to provide the required beam intensity. Hence the obtainedpattern sizes were considerably larger in these samples. However, byusing a high-resolution transmission electron microscope, a much smallertrough width can be achieved, as has been shown for GeS/Ag and GeSe₂/Agfilms by McHardy et al. These researchers produced silver-free troughsof 15 nm width.

[0066] The diffusion effect allows the production of a new generation ofx-ray masks with attainable pattern sizes of approximately 15 nm or lessdepending on the ability to focus the writing beam.

[0067] Currently x-ray masks are fabricated in a multistep process.Latest developments in producing x-ray masks, in which metal lines areproduced by decomposition of organometallic films upon electron beamexposure [xiv], reduce the number of steps involved but still requirethe dissolution of the remaining organometallic film after the exposure.Furthermore, the obtained pattern will be metallic, hence absorbingx-rays in the patterned areas, whereas the use of chalcogenide-metalsystems result in metal free patterns and therefore transmit the x-raysin the patterned areas.

[0068] From theoretical data [xv] it was determined that an As₂Se₃/Agfilm consisting of a 480 nm thick As₂Se₃ and a 500 nm thick silver film,which corresponds to 70 wt. % silver, exhibits a good contrast in thex-ray transmission curves at 1 keV, an energy that is commonly used inx-ray lithography.

EXAMPLE 5 Influence of Development of X-ray Masks in Alkaline Solutions

[0069] The photodoping effect, which has been observed inchalcogenide/silver films, occurs in a spectral range of about 300 to500 nm. X-rays would therefore not be able to induce the photodopingeffect, which in theory could reverse the patterning of the masks.

[0070] The investigated samples were successfully developed in analkaline solution (0.3N NaOH solution). This removed the chalcogenidefilm from the areas that had been patterned and did not contain silverany more. It was found that a development time of around 10 to 20 s wasenough to remove the chalcogenide films from the areas that were free ofsilver after the patterning. These patterns were then stable and thesilver could not diffuse back into these areas.

[0071] From these results it can be concluded, that a developmentprocedure can be employed to remove the chalcogenide layer in thepatterned areas. This can increase x-ray transmission when thickerchalcogenide films are used for the masks.

CONCLUSIONS

[0072] The silver diffusion effect in chalcogenide/silver thin films canbe exploited for the fabrication of x-ray masks. Providing suitable filmthickness and silver concentration are used, the patterns produced by ascanning electron beam are free of silver and transmit x-rays of certainenergy. The areas that were not exposed to electron beam irradiationcontain a sufficient amount of silver to absorb all incoming x-rays ofthat energy. For x-rays of 1 keV energy for example, the silver layershould be at least 500 nm thick to prevent transmission of these x-rays.

[0073] The thickness of the chalcogenide layer should be chosen so thata silver concentration of at least 30 wt. % is obtained. This is thelower limit at which the diffusion effect is noticeable. However, asilver concentration between 50 wt. % and 70 wt. % is preferred. Thesensitivity to electron beam irradiation is very high in samples withthese silver concentrations and mask writing efficiency can be improved.

[0074] To allow transmission of x-rays whilst absorbing x-rays in theareas unexposed to electrons the original sample should be produced asfollows. The chalcogenide layer should not be too thick. Generally alayer thickness of 480 nm is recommended to allow transmission of x-raysin the patterned areas. A silver film thickness of 500 nm whichcorresponds to 70 wt. % silver in the chalcogenide may be deposited, forexample, to guarantee absorption of x-rays in the non-exposed areas.

[0075] If lower silver concentrations were used, the chalcogenide layerwould have to be thicker and therefore would become less transparent to1 keV x-rays. In this case, a development procedure of the x-ray masksafter fabrication could be applied. This would remove the silver-freechalcogenide from the patterns and therefore increase the transmissionin these areas.

REFERENCES

[0076] [i] R. F. Pierret, Semiconductor Device Fundamentals,Addison-Wesley Publishing Company, 1996.

[0077] [ii] J. C. Kotz, P. Treichel, Jr., Chemistry & ChemicalReactivity, 3rd edition, Saunders College Publishing, 1996

[0078] [iii] A. E. Owen, A. P. Firth, P. J. S. Ewen, PhilosophicalMagazine B, Vol. 52

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[0081] [v] N. Yoshida, K. Tanaka, Journal of Applied Physics, Vol. 78(1995), No. 3, p. 1745

[0082] [vi] N. Yoshida, K. Tanaka, Journal of Non-crystalline Solids,210 (1997), p.119

[0083] [vii] N. Yoshida, M. Itoh, K. Tanaka, Journal of Non-CrystallineSolids, Vol. 198-200 (1996), p. 749

[0084] [viii] T. Kawaguchi, S. Maruno, K. Masui, Journal ofNon-Crystalline Solids, Vol. 77&78 (1985), p. 1141

[0085] [ix] N. Yoshida, K. Tanaka, Applied Physics Letters, Vol. 70(1997), No. 6, p. 779

[0086] [x] C. P. McHardy, A. G. Fitzgerald, P. A. Moir, M. Flynn, J.Phys. C: Solid States Phys., Vol. 20 (1987), p. 4055

[0087] [xi] KG. Fitzgerald, C. P. McHardy, Surface Science, Vol. 162(1985), p. 568

[0088] [xii] Intellemetrics, Deposition controller instruction manual,1987

[0089] [xiii] D. Sarid, Scanning Force Microscopy, Oxford UniversityPress, New York and Oxford, 1991

[0090] [xiv] G. J. Berry, J. A Cairns, M. R. Davidson, D. R. G. Rodley,J. Thomson, I. C. E. Turcu, W. Shaikh Review of Scientific Instruments,Vol. 69 (1998), No. 9, p. 3350

[0091] [xv] Web page of the Center for X-ray Optics, E.O. LawrenceBerkeley National Laboratory, URL: http://www-cxro.lbl.gov/

1. A method of preparing an x-ray photomask for use in the production ofsemi-conductor devices, the method comprising the steps of: a) providinga free-standing film of a radiation sensitive metal/chalcogenide; and b)exposing the film to an electron beam scanned in a defined pattern, inorder that metal in the metal/chalcogenide diffuses away on exposure tothe electron beam so as to generate areas in the film of reduced metalcontent in accordance with the defined pattern.
 2. The method accordingto claim 1 further comprising the step of: c) removing the chalcogenidein the electron-beam exposed area to leave a film comprisingmetal/chalcogenide in the unexposed areas and metal of reduced contentor substantially metal free content in the exposed areas.
 3. The methodaccording to either of claims 1 or 2 wherein the metal is silver,copper, platinum, palladium or any other metal which is able to reactwith the chalcogenide and diffuse away on exposure to a suitableradiation beam.
 4. The method according to any preceding claim whereinthe chalcogenide is selected from AS₂S₃, AS₂Se₃, GeS, GeSe, GeSe₂ andother non-stoichiometric chalcogenides of arsenic, germanium, bismuthand antimony.
 5. The method according to any preceeding claim whereinthe metal/chalcogenide film is greater than 30 wt %.
 6. The methodaccording to any preceding claim wherein features of less than 10 μmacross are capable of being patterned.
 7. The method according to anyone of claims 2-6 wherein the chalcogenide is removed using an alkalinesolution.
 8. The method according to claim 7 further comprising the stepof supporting the film on an x-ray transmissive substrate.
 9. Aphotomask prepared by the method according to any preceding claim.
 10. Aphotomask comprising an x-ray transmissive substrate comprising achalcogenide, supporting a metal film comprising areas of metal whichAreas are x-ray non-transmissive and areas of reduced metal content, orsubstantially metal-free which areas are x-ray transmissive.
 11. Use ofa photomask according to either of claims 9 or 10 in the manufacture ofan electronic device.